1. Field of the Invention
The present invention relates to a radio frequency amplifier which may, for example, be used in television, satellite and cable tuners.
2. Description of the Prior Art
It is a requirement of radio frequency amplifiers to be able to handle large amplitude signals while having a good noise figure so as to handle signals which may have a very large range of amplitudes. Various feedback techniques have been used to attempt to achieve this. Also, such amplifiers have generally been embodied by discrete components, which occupy a substantial area of printed circuit board and which are relatively expensive. It would thus be advantageous to be able to form such amplifiers in integrated circuits.
GB 2 295 934 discloses a radio frequency amplifier which uses positive feedback to boost its load resistance. The amplifier comprises an input stage whose output is connected to a load resistance. A transconductance amplifier has an input connected to the output of the input stage and an output connected to a resistor of the input stage so as to provide series negative feedback. The output of the input stage also supplies a further transconductance amplifier whose output is fed back to the load resistor to provide positive feedback in order to increase the effective resistance.
U.S. Pat. No. 5,661,437 discloses an arrangement for providing variable gain in a negative feedback amplifier. The amplifier comprises a common source FET gain stage with shunt feedback via a common drain FET. The gain is controlled by varying the bias voltage of the gate or drain of the feedback FET.
According to the invention, there is provided a radio frequency amplifier comprising: an input amplifier having an input for receiving input signals and an output; a first transconductance amplifier having an input connected to said output of said input amplifier and an output connected to provide series negative feedback to said input amplifier; and a second transconductance amplifier substantially identical to said first transconductance amplifier and having an input connected to said output of said input amplifier and an output forming an output of said radio frequency amplifier.
Said input amplifier may comprise a first resistor and first and second amplifying devices having common electrodes connected together by said first resistor, said output of said first transconductance amplifier being connected to said first resistor. Said first transconductance amplifier may comprise third and fourth amplifying devices having output electrodes connected to said common electrodes of said first and second amplifying devices. Said radio frequency amplifier may comprise first and second capacitors, said first and second amplifying devices having output electrodes and said third and fourth amplifying devices having input electrodes connected to said output electrodes of said first and second amplifying devices via said first and second capacitors, respectively. Said third and fourth amplifying devices may have input electrodes and said second transconductance amplifier may comprise fifth and sixth amplifying devices having input electrodes connected to said input electrodes of said third and fourth amplifying devices, respectively.
Said input amplifier may have a shunt negative feedback path to said input thereof Said first and second amplifying devices may have input and output electrodes and said shunt negative feedback path may comprise first and second voltage followers having inputs connected to said output electrodes of said first and second amplifying devices, respectively, and outputs connected to said input electrodes of said first and second devices, respectively.
The radio frequency amplifier may comprise a variable gain amplifier connected to said output of said second transconductance amplifier and having an output. The radio frequency amplifier may comprise a mixer connected to said output of said variable gain amplifier. Said mixer may comprise first and second inputs, first and second local oscillator inputs, first and second mixer outputs and seventh to tenth amplifying devices having common, control and output electrodes, said common electrodes of said seventh and eight amplifying devices being connected to said first signal input and said common electrodes of said ninth and tenth amplifying devices being connected to said second signal input, said control electrodes of said seventh and tenth amplifying devices being connected to said first local oscillator input and said control electrodes of said eight and ninth amplifying devices being connected to said second local oscillator input, said output electrodes of said seventh and ninth amplifying devices being connected to said first mixer output and said output electrodes of said eight and tenth amplifying devices being connected to said second mixer output. Said variable gain amplifier may comprise third and fourth signal inputs, first and second gain control inputs, first and second variable gain amplifier outputs, a power supply input and eleventh to fourteenth amplifying devices having common, control and output electrodes, said common electrodes of said eleventh and twelfth amplifying devices being connected to said third signal input and said common electrodes of said thirteenth and fourteenth amplifying devices being connected to said fourth signal input, said control electrodes of said eleventh and fourteenth amplifying devices being connected to said first gain control input and said control electrodes of said twelfth and thirteenth amplifying devices being connected to said second gain control input, said output electrodes of said eleventh and fourteenth amplifying devices being connected to said first and second variable gain amplifier outputs, respectively, and said output electrodes of said twelfth and thirteenth amplifying devices being connected to said power supply input.
The radio frequency amplifier may be formed in an integrated circuit.
It is thus possible to provide a radio frequency amplifier with an excellent noise figure and the capability of handling large input signal levels with good linearity so as to provide a wide dynamic range. The use of active boosted series feedback enhances the linearity of the amplifier without degrading the noise figure. The use of translinear techniques maintains the linearity over a large range of input signals. Further, the radio frequency amplifier may readily be made as an integrated circuit or as part of an integrated circuit. Thus, compared with known discrete component amplifiers, there is a saving in cost and a reduction in the required printed circuit board area.